Abstract
Abstract. Dry deposition is an important removal mechanism for tropospheric ozone (O3). Currently, O3 deposition to oceans in atmospheric chemistry and transport models (ACTMs) is generally represented using constant surface uptake resistances. This occurs despite the role of solubility, waterside turbulence and O3 reacting with ocean water reactants such as iodide resulting in substantial spatiotemporal variability in O3 deposition and concentrations in marine boundary layers. We hypothesize that O3 deposition to the Arctic Ocean, having a relatively low reactivity, is overestimated in current models with consequences for the tropospheric concentrations, lifetime and long-range transport of O3. We investigate the impact of the representation of oceanic O3 deposition to the simulated magnitude and spatiotemporal variability in Arctic surface O3. We have integrated the Coupled Ocean-Atmosphere Response Experiment Gas transfer algorithm (COAREG) into the mesoscale meteorology and atmospheric chemistry model Polar-WRF-Chem (WRF) which introduces a dependence of O3 deposition on physical and biogeochemical drivers of oceanic O3 deposition. Also, we reduced the O3 deposition to sea ice and snow. Here, we evaluate WRF and CAMS reanalysis data against hourly averaged surface O3 observations at 25 sites (latitudes > 60∘ N). This is the first time such a coupled modeling system has been evaluated against hourly observations at pan-Arctic sites to study the sensitivity of the magnitude and temporal variability in Arctic surface O3 on the deposition scheme. We find that it is important to nudge WRF to the ECMWF ERA5 reanalysis data to ensure adequate meteorological conditions to evaluate surface O3. We show that the mechanistic representation of O3 deposition over oceans and reduced snow/ice deposition improves simulated Arctic O3 mixing ratios both in magnitude and temporal variability compared to the constant resistance approach. Using COAREG, O3 deposition velocities are in the order of 0.01 cm s−1 compared to ∼ 0.05 cm s−1 in the constant resistance approach. The simulated monthly mean spatial variability in the mechanistic approach (0.01 to 0.018 cm s−1) expresses the sensitivity to chemical enhancement with dissolved iodide, whereas the temporal variability (up to ±20 % around the mean) expresses mainly differences in waterside turbulent transport. The mean bias for six sites above 70∘ N reduced from −3.8 to 0.3 ppb with the revision to ocean and snow/ice deposition. Our study confirms that O3 deposition to high-latitude oceans and snow/ice is generally overestimated in ACTMs. We recommend that a mechanistic representation of oceanic O3 deposition is preferred in ACTMs to improve the modeled Arctic surface O3 concentrations in terms of magnitude and temporal variability.
Highlights
Tropospheric ozone (O3) is the third most important greenhouse gas and a secondary air pollutant negatively affecting human health (Nuvolone et al, 2018) and plant growth (Ainsworth et al, 2012) due to its oxidative character
Using a much higher spatial and temporal resolutions compared to other global modeling studies, we aim to evaluate to what extent the role of spatiotemporal variability in O3 deposition explains observed surface O3 concentrations regarding temporal variability
We find that the nudged to ERA5 synoptic conditions (NUDGED) run (Fig. 3a) systematically underestimates the mean observed surface O3 mixing ratios for the High Arctic sites by ∼ 5–10 ppb, which appears to be caused by an overestimated deposition to ocean, snow and ice surfaces, further substantiated by the following analysis of temporal variability in O3 concentrations (Sect. 3.3)
Summary
Tropospheric ozone (O3) is the third most important greenhouse gas and a secondary air pollutant negatively affecting human health (Nuvolone et al, 2018) and plant growth (Ainsworth et al, 2012) due to its oxidative character. Understanding the Arctic O3 budget is of particular interest because its remote location implies that anthropogenic sources and sinks are generally absent This implies that these Arctic O3 observations allow us to determine large-scale trends in tropospheric O3 (Helmig et al, 2007b; Gaudel et al, 2020; Cooper et al, 2020). Local emissions of precursors are expected to become an important source of Arctic O3 concentrations due to the warming Arctic climate and increasing local economic activity (Marelle et al, 2016; Law et al, 2017) This underlines the need for understanding the sources and sinks of Arctic tropospheric O3 and to accurately representing them in atmospheric chemistry and transport models (ACTMs)
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